I have recently stumbled upon this library which I find having a great potential. Cheaper Nucleo controller and power boards help to prototype greatly without having to rely on expensive vendors boards like TI for example. I am using r2019b, code gen...

Try to enable the GPIO pin that you are intending to use as 'GPIO_Output' in the <.ioc> file in CubeMX. Then save it. In Simulink update the model (Ctrl + D).Then you should be able to use the pin.